The present invention relates to a vacuum degassing apparatus for molten glass, which removes bubbles from molten glass continuously supplied.
In order to improve the quality of formed glass products, there has been used a vacuum degassing apparatus which removes bubbles generated in molten glass before the molten glass which has been molten in a melting vessel is formed by a forming apparatus, as shown in FIG. 4.
The vacuum degassing apparatus 410 shown in FIG. 4 is used in a process wherein molten glass G in the melting vessel 420 is vacuum-degassed and is continuously supplied to a subsequent treatment vessel. The vacuum degassing apparatus includes a vacuum housing 412 which is evacuated to be depressurized therein for vacuum-degassing, a vacuum degassing vessel 414 which is depressurized along with the vacuum housing 412, and an uprising pipe 416 and a downfalling pipe 418 which are connected to both end portions of the vacuum degassing vessel so as to extend downwardly and vertically. The uprising pipe 416 has a lower end immersed in the molten glass G in an upstream pit 422 in communication with the melting vessel 420. Likewise, the downfalling pipe 418 has a lower end immersed in the molten glass G in a downstream pit 424 in communication with the subsequent treatment vessel (not shown).
The vacuum degassing vessel 414 is provided in the vacuum housing 412 in a substantially horizontal direction, which is evacuated by a vacuum pump, not shown, to be depressurized therein. The vacuum degassing vessel 414 as well as the vacuum housing 412 is depressurized to a pressure of {fraction (1/20)}-⅓ atmosphere therein. As a result, the molten glass G before vacuum-degassing in the upstream pit 422 is sucked and drawn up by the uprising pipe 416 to be introduced into the vacuum degassing vessel 414, the molten glass is vacuum-degassed in the vacuum degassing vessel 414, and the molten glass is drawn down by the downfalling pipe 418 to be discharged into the downstream pit 424.
In order to evacuate the vacuum housing 412 through a suction port 412c by the vacuum pump or the like, not shown, to depressurize the inside of the vacuum degassing vessel 414 to a certain pressure and to maintain the certain pressure therein, the vacuum degassing vessel 414 has suction ports 414a, 414b formed in an upper portion thereof so as to be open toward the inside of the vacuum housing 412.
The vacuum housing 412 is a metallic casing, such as an ordinary steel casing, a stainless steel casing or a heat-resistant steel casing. The vacuum housing is evacuated by the vacuum pump (not shown) or the like from outside to be depressurized therein, depressurizing the inside of the vacuum degassing vessel 414 provided therein to the certain reduced pressure, such as a pressure of {fraction (1/20)}-⅓ atmosphere and maintaining the reduced pressure in the vacuum degassing vessel.
Around the vacuum degassing vessel 414, the uprising pipe 416 and the downfalling pipe 418 in the vacuum housing 412 is provided thermal insulation material 430, such as refractory bricks, to cover these members for thermal insulation.
Since the conventional vacuum degassing apparatus 410 is used to deal with the molten glass G at a high temperature, such as a temperature in a range of 1,200-1,400xc2x0 C., paths for molten glass in direct contact with the molten glass G, such as the vacuum degassing vessel 414, the uprising pipe 416 and the downfalling pipe 418, are constituted by circular shells made of noble metal, such as platinum and platinum alloy like platinum rhodium, as disclosed in JP-A-2221129 in the name of the applicants.
The reason why the paths for molten glass, such as the vacuum degassing vessel 414, the uprising pipe 416 and the downfalling pipe 418, are constituted by a circular shell made of noble metal, such as platinum and platinum alloy, is that there is no inclusion of impurities into the molten glass G and a certain strength is ensured at high temperatures since it is hardly possible due to low reactivity of the noble metal with the molten glass at a high temperature that, when the noble metal contacts the molten glass G at such a high temperature, the noble metal does not elute by reaction with the molten glass G.
When the paths for molten glass, such as the vacuum degassing vessel 414, the uprising pipe 416 and the downfalling pipe 418, are made of noble metal, such as platinum and platinum alloy, the following problems are created especially in building the vacuum degassing apparatus 410 in a large size:
(1) It is required that the temperature of the molten glass G at an inlet of the vacuum degassing apparatus 410 be not higher than 1,400xc2x0 C.
When the temperature is higher than 1,400xc2x0 C., the strength of the noble metal lowers. In order to set the temperature at a temperature not higher than 1,400xc2x0 C., the temperature in the melting vessel 420 can not be raised in a sufficient way. This could lead to insufficient 25 dissolution of glass material in the melting vessel 410.
(2) The cost greatly increases.
In order to deal with a large quantity of molten glass, the paths are required to have a large sectional area. This requires that the paths have a thick wall to maintain a required strength for the paths, remarkably increasing the required quantity of the expensive noble metal and significantly raising the cost.
In terms of mainly cost reduction, a proposal has been made to constitute the paths for molten glass in the vacuum degassing vessel 414, the uprising pipe 416 and the downfalling pipe 418 in the conventional vacuum degassing apparatus 410 shown in FIG. 4 by refractory bricks more inexpensive than noble metal, such as platinum and platinum alloy, in order to build the apparatus in a large size and increase the degassing throughput of the molten glass.
However, there are limits to which refractory bricks are formed in a large size. It is absolutely impossible to build each of the vacuum degassing vessel 414, the uprising pipe 416 and the downfalling pipe 418 from a single refractory brick. In order to constitute the vacuum degassing vessel 414, the uprising pipe 416 and the downfalling pipe 418 of the vacuum degassing apparatus 410 by refractory bricks, many refractory bricks are required to be combined. This means that joints are inevitably formed between the refractory bricks in the paths in direct contact with the molten glass.
If a joint is in touch with atmosphere, there is a possibility that air enters into the paths through the joint to make it difficult to maintain the depressurized state in the paths since the inside of the paths in direct contact with the molten glass is depressurized. There is no possibility that this problem is created at the vacuum degassing vessel 414 because the vacuum degassing vessel is housed in the depressurized vacuum housing 412. However, there is left a possibility that air enters through a joint because it is difficult to house the uprising pipe 416 and the downfalling pipe 418 in their entirety in the vacuum housing.
It is an object of the present invention to solve the problems of the conventional apparatus and to provide a large size of practical vacuum degassing apparatus capable of dealing with a large quantity of molten glass.
In order to attain the object, the present invention provides a vacuum degassing apparatus for molten glass, comprising a vacuum housing which is evacuated to be depressurized therein; a vacuum degassing vessel made of refractory material, which is provided in the vacuum housing to vacuum-degas molten glass; an uprising pipe made of refractory material, which connects to the vacuum degassing vessel in the vacuum housing, and sucks and draws up undegassed molten glass to introduce the undegassed molten glass into the vacuum degassing vessel; a downfalling pipe made of refractory material, which connects to the vacuum degassing vessel in the vacuum housing and draws down the degassed molten glass from the vacuum degassing vessel to discharge the degassed molten glass; an upstream pit which supports the vacuum housing and the uprising pipe and connects to the uprising pipe; and a downstream pit which supports the vacuum housing and the downfalling pipe and connects to the downfalling pipe; wherein the uprising pipe and the downfalling pipe have lower edges positioned at a lower level than a liquid level of the undegassed molten glass when depressurization starts.
It is preferable that there is provided a brick receiver between the uprising pipe and the upstream pit, and/or a brick receiver between the downfalling pipe and the downstream pit.
It is preferable that the brick receiver comprises a plurality of refractory bricks combined so as to have polished surfaces located in surface contact with each other, the polished surfaces having a smoothness of not greater than 0.3 mm, and seal glass is filled into a joint between adjoining refractory bricks. It is preferable that the brick receiver has a metallic retainer provided therearound.
It is preferable that the vacuum housing has a lower end portion extended downwardly along the uprising pipe (the downfalling pipe), the lower end portion has a lower end located at the same level as the lower end of the uprising pipe (the downfalling pipe) in a vertical direction, the brick receiver is interposed between the lower end portion and the uprising pipe (the downfalling pipe), and the upstream pit (the downstream pit), and the brick receiver has sealing material filled in a portion thereof in contact with the lower end portion. It is more preferable that the lower end portion has a water pipe provided therearound.
It is preferable that at least one of the uprising pipe and the downfalling pipe has a lower end portion extended from the vacuum housing, and at least one portion around an outer periphery of the lower end portion of the at least one of the uprising pipe and the downfalling pipe is covered by sealing material. It is more preferable that a water pipe is provided to contact with the sealing material.
It is preferable that the lower end portion of the at least one of the uprising pipe and the downfalling pipe is clamped by a band-shaped metallic fixture on an outer surface of the sealing material. It is particularly preferable that the water pipe is provided between the sealing material and the band-shaped metallic fixture.
It is preferable that at least one of a contacting portion of the lower end portion of the uprising pipe and a brick receiver interposed between the uprising pipe and the upstream pit, and a contacting portion of the lower end portion of the downfalling pipe and a brick receiver interposed between the downfalling pipe and the downstream pit has sealing material filled therein.